1. Field of the Invention
In the manufacture of flexible film packages by means of heat seal methods, there has long been a need for heat seal compositions exhibiting a broad enough heat sealing temperature range that both permanent and peelable seals can be made reproducibly in the same packages with a variety of film materials. The permanent seals in such packages are preferably so strong that seal failure will occur by tearing the packaging film; and the peelable seals are preferably strong enough to hold contents of the package but weak enough to permit separation of the package walls with a moderate force. Rupture of the peelable seals is preferably a cohesive failure in the heat seal composition itself; and the rupture is preferably clean without a loose debris of the heat seal composition. A broad temperature range between formation of the peelable seals and the permanent seals is important because, with a broad temperature range, a single heat sealing device can be used to make peelable and permanent seals without need for highly sensitive thermostatic controls to be built into the device.
2. Description of the Prior Art
U.S. Pat. No. 4,188,441, issued Feb. 12, 1980, discloses a blend of polymers useful with high density polyethylene film to make peelable heat seals. The blend of that patent is disclosed to be of an ethylene vinyl acetate or acrylic ester two-component copolymer and a sodium-neutralized ionomer. The disclosed object is to make peelable seals only and permanent seals are avoided as undesirable.
U.S. Pat. No. 3,707,393 issued Dec. 26, 1972 on the application of McDonald discloses heat seal use of a combination of an ionomer and, in a special instance, a terpolymer. The combination of materials is merely a physical mixture and not a blend with partial neutralization of both polymers.
U.S. Pat. No. 3,789,035 issued Jan. 29, 1974 on the application of Iwami et al. discloses a process wherein carboxylic acid groups in one polymeric material can be neutralized by reaction contact with neutralized carboxylic acid groups in another polymeric material. The patent also discloses separation of the polymers so neutralized.
U.S. Pat. No. 3,496,061 issued Feb. 17, 1970 on the application of Freshour et al. discloses heat sealed packages made using at least two different materials so that at least one seal will exhibit a peel strength of more than 400 grams per inch.
U.S. Pat. No. 4,189,519 issued Feb. 19, 1980 on the application of Ticknor discloses a blend of polymeric materials which serves as a heat seal composition wherein one required compound is an isotactic polybutylene.